ABSTRACT Unlike adults, young children exhibit increased susceptibility to a variety of bacterial pathogens that express capsular polysaccharides (PS) such as Salmonella enterica serovar Typhi (S. Typhi). Vaccination of children is considered the most promising strategy for prevention of such childhood infections. However, purified bacterial PS vaccines such as the ViPS of S. Typhi do not induce an antibody response in young children. Conjugate vaccines (bacterial PS conjugated to protein carriers) induce B cell responses in young children but require many doses administered months apart for generating optimal levels of protective antibody. This makes adherence to vaccination schedules, particularly in remote areas difficult. In the case of ViPS conjugate vaccines for S. Typhi, recent clinical trials in several typhoid endemic countries have been disappointing. We have utilized the NOD/SCID/common cytokine receptor ? chain knockout (NSG) strain of mice and human hematopoietic stem cells (HSCs) to create hematopoietically humanized mice (HISmice) to allow experimental analysis of human immune responses in vivo to infectious pathogens of relevance to humans. We have found that these mice are reproducibly and stably engrafted with human B lymphocytes in the blood, spleen, peritoneal cavity, mesenteric lymph node and gut associated lymphoid tissue. However, major subpopulations of the B cell compartment in these mice appear phenotypically immature. Moreover, while HISmice created via our current protocol produce robust antibody responses to some bacteria, but like young children they do not respond to ViPS and other purified capsular PS antigens. Others have found that HISmice, unlike conventional mice are susceptible to infection by S. Typhi. These observations lead us to hypothesize that HISmice generated via our current protocol fail to respond to PS antigen such as ViPS due to the fact that their B cell compartments mimic those of young children. We hypothesize that this is due, in part, to lack of efficient cross stimulation of human cytokine receptors on engrafting hematopoietic cells by host-produced murine cytokines. In this application, we propose to test this hypothesis in the case of interleukin-7 (IL7), using S. Typhi as a particularly relevant human pathogen. IL7 plays a central role in the development of B cells and B cell antigen receptor repertoire in mice and has been suggested to sub serve an analogous function in humans. We will exploit the recent development of adeno- associated virus (AAV) gene therapy approaches that allow facile expression of human cytokines in NSG mice. We predict that expression of human IL7 will yield HISmice with more mature and diverse B cell compartments capable of mounting robust immune responses to ViPS resulting in protection from S. Typhi challenge. As such, these studies will provide critical new data useful in the development of vaccination strategies for the prevention of S. Typhi and other childhood diseases caused by encapsulated bacteria.